Exemplary embodiments relate generally to imaging devices, and more particularly to imaging devices with a plurality of imagers that provide sequential images that are overlaid to form a composite image.
Imaging devices often utilize a first color to produce a first image, portions of which are desired to be highlighted using a second color. In order to produce the desired results, the imaging device must precisely register the highlighted color image with the first image.
Highlight color image registration is often challenging. It is often the case that a highlight printer is designed as a retrofit of a monochromatic engine in which the quality of the motion of the photoreceptor is only sufficient to limit the banding to a tolerable level. The monochromatic image is typically laid down at a constant rate of lines per unit time by a first imager. If a second imager (to highlight the first image using a second color) is also caused to write at a constant rate, serious errors in color to color registration may occur.
In single pass electrophotographic printers having more than one process station that provide sequential images to form a composite image, critical control of the registration of each of the sequenced images is required. This is also true in multiple pass color printers, which produce sequential developed images superimposed onto a photoreceptor belt for charging with toner to form a multi-color image. Failure to achieve registration of the images results in printed copies in which the color separations forming the images are misaligned. This condition is generally obvious upon viewing of the copy because such copies usually exhibit fuzzy color separation between color patches, bleeding and/or other errors. These errors may make such copies unsuitable for intended uses.
A typical highlight color reproduction machine records successive electrostatic latent images on the photoconductive surface. One latent image is usually developed with black toner. The other latent image is developed with color highlighting toner, e.g., red toner. These developed toner powder images are transferred to a sheet to form a color-highlighted document. When combined, these developed images form an image corresponding to the entire original document being printed. This color highlighting reproduction machine can be of the so-called single-pass variety, where the color separations are generated sequentially by separate imaging and toning stations, or of the so-called multiple-pass variety, where the separations are generated by a single imaging station in subsequent passes of the photoreceptor and are alternatively toned by appropriate toning stations.
A particular variety of single-pass highlight color reproduction machines using tri-level printing have also been developed. Tri-level electro-statographic printing is described in greater detail in U.S. Pat. No. 4,078,929. As described in this patent, the latent image is simultaneously developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged, and the toner particles of the other color are negatively charged.
Another type of color reproduction machine which may produce highlight color copies initially charges the photoconductive member. Thereafter, the charged portion of the photoconductive member is discharged to form an electrostatic latent image thereon. The latent image is subsequently developed with black toner particles. The photoconductive member is then recharged and exposed to record the highlight color portions of the latent image thereon. A highlight latent image is then developed with toner particles of a color other than black, e.g., red, and then developed to form the highlight latent image. Thereafter, both toner powder images are transferred to a sheet and subsequently fused thereto to form a highlight color document.
Exemplary operations of highlight and color printers are described in greater detail in U.S. Pat. Nos. 5,113,202, 5,208,636, 5,281,999 and 5,394,223, the disclosures of which are hereby incorporated herein by reference in their entireties.
A hybrid reflex writing printer is described in commonly-owned U.S. patent application Ser. Nos. 10/909,075 and 11/289,888, which are also incorporated herein by reference in their entireties.